Fracturing is widely used to increase the productivity of oil and gas wells. This is generally accomplished by forcing liquids into the well under pressure to open cracks in the formations surrounding the well. Various solid materials are then introduced to prop open the fractures that are formed.
Sand is frequently used as a propping agent in such applications. It is relatively inexpensive, and when suspended in liquids it is readily carried into the fractures. However, if the well is deep, the high pressures encountered crush some of the sand giving finely-divided particles which tend to plug the fracture. For this reason, alternate propping agents have been sought for use in deep wells.
One propping agent recently found to be suitable for use in deep wells is sintered bauxite. Although this material is able to withstand the high pressures developed in the wells, it is comparatively expensive and it can undergo attrition by chemical action in some applications. Consequently, less costly substances have been sought. In particular, many workers have attempted to treat sand and other solid particles to enable them to withstand the high pressures found in deep wells.
One metbod for treating proppant sand is that disclosed in U.S. Pat. No. 3,026,938. According to this process, sand particles are coated with a flexible material such as rubber latex. The resulting particles are compressed and not broken when subjected to high pressure in the fracture. However, this material has found limited use in formations subjected to high pressure because this proppant fails to maintain good permeability when compressed.
Another type of coated sand is disclosed in U.S. Pat. No. 3,929,191. According to this disclosure, sand is coated with an uncured, thermoplastic resin which first melts and then cures in the fracture. Such a process causes the sand grains to be bound together preventing their movement out of the fracture. However, for these products to be satisfactory, curing must be carefully controlled. If the coating cures too rapidly, it may cause plugging of the well bore or aggregation of the sand before it reaches the extremities of the fracture. If the coating cures too slowly, the well has to be held under pressure until the curing is completed.
A process for making a propping agent coated with a cured epoxy resin is disclosed in U.S. Pat. No. 3,935,339. According to this process, a mixture of solid particles, uncured epoxy resin, a surfactant, and a heated liquid in which the resin is insoluble is agitated until the coating cures. The coated solid is then separated from the liquid.
A similar process for obtaining a propping agent coated with a cured furan resin is disclosed in U.S. Pat. No. 3,492,147. According to this process, particulate matter, coated with furfuryl alcohol or uncured furfuryl alcohol resin, is stirred in heated oil containing an acid catalyst to give individual particles coated with an infusible resin. An alternate process which involves mixing the solid with an uncured resin solution and an acid catalyst solution in a continuous ribbon blender and continuously routing the mixture into a heated chamber is also mentioned.
The present invention is directed to a method for the preparation of particulate matter uniformly coated with a cured phenolic resin which does not require the expensive process of curing the product in an inert solvent. By this process, the particles are readily coated with a cured resin without the formation of large amounts of agglomerated material.
The products of this invention are resistant to crushing when subjected to high pressures, giving proppants of high permeability. Their coating makes them resistant to attack by acids and steam, materials often used in treating wells. Finally, their lubricated coating is resistant to abrasion reducing the formation of dust which could cause plugging of well openings. Such properties make these products useful not only in hydraulic fracturing but also in gravel pack completions and other applications as well.